1. Field of the Invention
This invention relates generally to geophones used in seismic exploration.
2. Description of the Prior Art
FIG. 1 shows a vertical geophone (10) of conventional design. FIG. 1 is a cross section taken along the longitudinal axis of the geophone (10). The geophone (10) employs a cylindrical magnet (14), cylindrical upper and lower ferrous pole pieces (16, 18), and a tubular ferrous outer housing (20), which together form a magnetic circuit.
The upper and lower pole pieces (16, 18) each have a cap-like shape so that they fit over and receive the upper and lower ends of the magnet (14), respectively. The tubular portion of the upper and lower pole pieces that enclose the sides of the cylindrical magnet (14) are referred to herein as pole piece lips (80, 82). The magnet (14) and pole pieces (16, 18) are received within the outer cylindrical housing (20). In addition to shaping magnetic flux across the air gaps (22, 24), as described below, the circular lips (80, 82) function to keep the magnet (14) precisely coaxially aligned between the pole pieces (16, 18).
The lower pole piece (18) and the outer housing (20) are connected to a lower end cap (26), which is in turn connected to the earth, such as by a stake (not shown) that is placed within the ground, so as to transmit seismic vibrations to the magnet and pole pieces. The lower end cap (26) is typically formed of an electrically non-conductive plastic material. An upper end cap (28) is connected between the upper pole piece (16) and the upper end of the outer housing (20). The upper end cap (28) is also typically made of a dielectric plastic material. The upper and lower end caps (28, 26), also referred to as headers, are held within the cylindrical housing (20) by crimping or swaging the upper and lower perimeters of the housing (20) about the end caps.
The bottom end of the lower pole piece (18) includes a centered circular recess (84) that mates with a centered circular protrusion (85) formed on the upper side of the lower end cap (26), thereby precisely coaxially aligning the lower pole piece (18) with the lower end cap (26). As an equivalent alternative (not illustrated), the lower pole piece (18) may have a circular protrusion that is received into a circular recess formed in the lower end cap (26). A similar arrangement of a circular recess (86) and a circular protrusion (87) keeps the upper pole piece (16) coaxially aligned with the upper end cap (28).
Within the annular space between the magnet (14) and pole pieces (16, 18) on the one hand, and the housing (20) on the other, an inertial member, typically a cylindrical coil form (30), is suspended between an upper frequency-tuned spring (32) and a lower frequency-tuned spring (34). The frequency-tuned springs (32, 34) allow the magnet (14), pole pieces (16, 18), and outer housing (20) to vibrate up and down coaxially with respect to the coil form (30) while the coil form (30) remains essentially motionless and decoupled from the rest of the geophone (10). The frequency-tuned springs (32, 34) are designed and tuned to provide a desired resonant frequency.
The upper and lower frequency-tuned springs (32, 34), also known as spider springs, are typically washer-shaped. The inner circumferences of the frequency-tuned springs (32, 34) are sandwiched between their respective end caps (28, 26) and pole pieces (16, 18). The outer circumferences of the frequency-tuned springs (32, 34) are connected to the upper and lower ends of the coil form (30) as described in greater detail below with respect to FIG. 2. The upper and lower frequency-tuned springs (32, 34) fit precisely about the upper and lower circular inward-facing protrusions (87, 85) of the upper and lower end caps (28, 26), respectively, thereby coaxially centering the coil form (30).
FIG. 2 is an enlarged view of the upper frequency-tuned spring/coil form interface as indicated in FIG. 1. Referring to FIG. 2, the upper end of the coil form (30) forms an interior circumferential groove (36) and an upward-facing ledge (37). The outer circumferences of the upper frequency-tuned spring (32) is disposed within the interior circumferential groove (36) and seated on the upward-facing ledge (37). A C-ring, C-clip, or like clamp (38) is then compressed and inserted into the groove (36). The C-ring clamp (38) continually exerts a radial force against the coil form (30), which prevents it from coming out of the groove (36) thereby securing the upper frequency-tuned spring (32) to the upper end of the coil form (30). A similar arrangement secures the outer circumference of the lower frequency-tuned spring (34) to the lower end of the coil form (30).
FIG. 3 is an enlarged side view cross-section of a prior art geophone that illustrates an alternate arrangement for mounting the frequency-tuned springs to the coil form according to U.S. Pat. No. 3,738,445, issued to Wilson et al. and entitled “Seismometer Spring Suspension System.” An upper frequency spring (32′) that includes openings (33) formed therethrough about its circumference is illustrated. The upper frequency spring (32′) is mounted to the upward-facing ledge (37′) of the coil form (30′) by a thin layer of adhesive (91). The adhesive is also disposed through the openings (33), and once cured forms solid columns of adhesive (93). Because the coil form (30′) has no recess into which the upper frequency spring (32′) is received, the upper frequency spring (32′) is not self-centering with respect to the coil form (30′). The layer of adhesive (91) between the coil form ledge (37) and the upper frequency spring (32′) may affect the resonance of the spring. Moreover, before it adhesive cures, the adhesive (91) may drip on to the inside and outside walls of the coil form (30′), which may be difficult to clean.
Referring back to FIG. 1, the geophone (10) forms a magnetic circuit with upper and lower annular air gaps (22, 24) that are defined in the regions between the upper and lower pole pieces (16, 18), respectively, and the housing (20). A magnetic flux is created by and passes axially through the magnet (14). From the upper end of the magnet (14), the magnetic flux is channeled and redirected through the upper pole piece (16) so as to pass radially across the upper annular air gap (22) to the housing (20). The flux passes vertically downward through the cylindrical housing (20) and radially inward to the lower pole piece (18) through the lower air gap (24). The lower pole piece channels and redirects the flux to the lower end of the magnet (14) to complete the magnetic circuit.
Upper and lower electrical coils (40, 42) are wound about the coil form (30) so as to be located in the upper and lower air gaps (22, 24), respectively. The winding direction of the upper coil (40) is opposite of the winding direction of the lower coil (42), and the coils are electrically connected together in series.
An electrical circuit is formed as follows: The upper lead (not visible) of the upper coil (40) is electrically connected to the outer circumference of the upper frequency-tuned spring (32) by a solder joint, for example. The inner circumference of the upper frequency-tuned spring (32) makes sliding electrical contact with an outer wiper ring assembly (61), which includes a lead (60) that passes through the upper end cap (28). The inner circumference of the upper frequency-tuned spring (32) is separated and electrically isolated from the upper pole piece (16) by a thin dielectric washer (52) that is positioned therebetween. The lower lead (not illustrated) of the upper coil (40) is connected to the upper lead (not illustrated) of the lower coil (42).
The lower lead (not visible) of the lower coil (42) is connected to the outer circumference of the lower frequency-tuned spring (34) by solder joint, for example. The inner circumference of the lower frequency-tuned spring (34) makes sliding electrical contact with the lower surface of the lower pole piece (18). An electrical connection is formed between the lower pole piece (18) and the upper pole piece (16) through abutting contact of the upper and the lower pole pieces with the magnet (14). Finally, the upper pole piece (16) makes sliding electrical contact with an inner wiper surface (63), which includes a lead (64) that passes through the upper end cap (28).
The first and second leads (60, 64) are connected to geophone recording circuitry through a seismic cable (not illustrated). The arrangement of this electrical circuit allows the coil form (30) to freely rotate about its vertical axis within the geophone (10), thus minimizing the possibility of damage from rough handling.
In operation, a terrestrial vibration causes the magnetic circuit components, and hence the magnetic flux, to vibrate up and down relative to the coil form (30), which remains essentially stationary due to its inertia. As the radial flux lines cut the upper and lower coils (40, 42), changes in flux density induce an electromotive force in the coils according to Faraday's law. This induced voltage is measured at the first and second leads (60, 64) via the electrical circuit described above.
Damping of the coil form (30) is necessary so that there will not be continual oscillation of the coil form (30) relative to the rest of the geophone (10), but too much damping reduces geophone sensitivity. Damping of the coil form (30) is a function of both the mass and the electrical conductivity of the coil form (30). The conductivity affects the formation of eddy currents in the coil form (30) created by Faraday induction. The coil form eddy currents flowing in a magnetic field result in a force being exerted on the coil form (30) that opposes the motion that created the eddy currents. Conductivity of the prior art coil form (30) is controlled by using a two-part coil form assembly. The coil form (30) includes an upper bobbin sleeve (70) that is received into a lower bobbin sleeve (72) (or vice versa). The upper and lower bobbin sleeves (70, 72) are formed of anodized aluminum and are joined by adhesive. The anodization layer electrically insulates the bobbin halves, thereby reducing the formation of eddy currents in the assembled two-piece coil form (30) as compared to a one-piece coil form of identical dimensions.
In conducting a seismic survey, multiple geophone channels are recorded. As computing power increases, it has become more desirable to conduct high resolution surveys across large geographical areas, which necessitates that large number of geophone channels are employed in a given survey. Therefore, it is desirable to reduce the geophone size, thus decreasing the overall capital and operational cost of the survey system. For example, when geophones are used in marine streamers, smaller geophone size allows streamer diameter to be reduced, which in turn allows longer streamers to be employed with greater members of geophones per cable.
The process of miniaturizing a geophone of prior art, such as the geophone (10) of FIG. 1, is not merely an exercise in scaling, because as the size of the magnet is reduced, the ability to output a voltage signal that is detectable above the ambient noise level is diminished. Accordingly, it is desirable to decrease geophone dimensions while at the same time maximizing the magnet size.
The process of miniaturizing a geophone also inherently alters the natural frequency response of a geophone. Substantial reductions in geophone size while providing an acceptable frequency response have heretofore not been possible.
Additionally, limitations in manufacturing processes and materials have also been a factor in heretofore preventing the design of a micro-geophone that can be manufactured with a commercially reasonable cost.
3. Identification of Objects of the Invention
A primary object of the invention is to provide a significantly miniaturized geophone having the sensitivity and frequency response of the much larger traditional geophones.
Another object of the invention is to provide a method and apparatus for precisely controlling geophone damping by tightly controlling the overall mass of a geophone coil/coil form assembly.
Another object of the invention is to provide a miniaturized geophone characterized by a frequency response tuned for frequencies of 30 Hertz or lower.